The present invention relates generally to an organic EL (electroluminescent) device using an organic compound, and more specifically to an improvement in or relating to a sealing sheet used therewith
In recent years, organic EL devices have been under intensive investigation. One such device is basically built up of a thin film form of hole transporting material such as triphenyldiamine (TPD) deposited by evaporation on a hole injecting electrode, a light emitting layer of fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) laminated thereon, and a metal (electron injecting) electrode of a metal having a low work function such as Mg and formed on the light emitting layer. This organic EL device now attracts attentions because a very high luminance ranging from several hundreds to tens of thousands cd/m.sup.2 can be achieved with a voltage of approximately 10 V.
A grave problem with an organic EL device is that it is acutely sensitive to moisture. Exposure of the organic EL device to moisture, for instance, causes a light emitting layer to peel off electrode layers, or materials forming part thereof to degenerate. This results in another problem that non-light emission spots called dark spots occur or any consistent light emission cannot be kept.
One approach known to solve this problem is to fix an airtight case, a sealing layer or the like closely on a substrate to cover an organic EL multilayer structure, thereby shielding it from the outside, as typically disclosed in JP-A's 5-36475, 5-89959 and 7-169567.
However, the provision of such a sealing layer in a box form is not suitable for the mass production of organic EL devices because of a production cost increase. For such a sealing layer, inexpensive glasses are usually employed. When a glass sheet is used with no modification thereto, however, it is likely to come into contact with, or abutment on, an organic EL multilayer structure (as seen in FIG. 3), resulting in a breakdown of the organic EL structure. In an effort, a spacer or the like is used to bond a glass sealing sheet to an organic EL structure while the glass sealing sheet is kept at a height higher than the organic EL structure. Even when the portion of the glass sealing sheet bonded to the organic EL structure is kept at the required height, however, it is likely that the undulations of the glass sealing sheet and a substrate with an organic EL device stacked thereon cause the glass sealing sheet to come into contact with, or abutment on, an organic EL device structure, resulting in a breakdown of the organic EL structure or the occurrence of dark spots. When the sealing sheet is kept at a height higher than required, on the other hand, a problem arises in conjunction with airtightness, and no suitable spacer is available.